It has some compatibility issues, but not enough to knock it down to a four! It is one of the few extensions which I would be disinclined to browse without. It reduces the # of windows open at once. It also has a mastery of chrome:// urls, enabling view of various things in tabs (bookmarks, downloads, addons, error console, history, etc.), in case sidebar real estate is cramped. Should be standard on Fx3, along with AdBlock+ (AB+ isn\'t AFAIK, but they both should be).